Mikhail S. Gorbachev said in a letter to President Oscar Arias of Costa Rica that the Soviet government was prepared to stop sending arms to Central America if the United States agreed to do the same.

In the letter, the Soviet Communist Party general secretary also said he had proposed to the United States to embargo all arms shipments to the region, except for firearms for police forces.The Soviet Union is a major arms supplier to Nicaragua's Sandinista government.

Reports that the Soviet Union was sending arms to leftist Salvadoran and Guatemalan guerrillas were "totally unfounded," said the letter, which was made public Saturday by Arias' press office.

Gorbachev said his government was conducting its relations with Central America's governments in accordance with the regional peace plan authored by Arias and signed by the five Central American presidents on Aug. 7.

That plan calls for an end to outside assistance for insurgencies in the region, cease-fires in Nicaragua, El Salvador, and Guatemala, and talks aimed at establishing democratic elections in the region.